Microwave Chocolate Cake Pudding

To make the chocolate sponge pudding, place the soft butter and sugar in a large mixing bowl. Using an electric mixer or wooden spoon, cream the butter and sugar together very well until very light and fluffy.
Beat in the eggs, one at a time and beat well before adding the next to prevent the mixture from curdling. Beat in the vanilla extract.
Sift together the flour, cocoa powder and salt. Fold the dry ingredients into the creamed butter and sugar until the batter is smooth. Fold in the milk. You should have a light chocolate batter that falls easily from a spoon (this is called "dropping consistency".)

Spoon the sponge batter into a greased 1 1/2 pint microwave safe pudding bowl and smooth the top. Microwave for 3-4 minutes or until the sponge is light and springy to the touch and there is no trace of raw batter. It might look a little wet in parts, but that's fine. A little stickiness is desirable here. Leave the pudding to stand in the microwave for 2-3 minutes before turning out onto a serving plate.

To make the chocolate custard, beat the egg yolks and sugar together well, then whisk in the cornflour, vanilla extract and cocoa powder.
Bring the milk to the boil in a saucepan, then immediately remove from the heat and gradually pour the hot milk into the egg mixture, whisking all the time. I find it helpful to place the bowl of egg yolks on a tea towel so it's not dancing all over my worktop when I'm trying to whisk in the milk!
Pour the mixture back into the saucepan and cook gently over a low heat for about 5-8 minutes, or until the custard has thickened.

Serve sauce with the chocolate sponge straight away.